The present invention relates to generally a piston valve device for producing the damping force in a hydraulic shock absorber and more particularly an improvement of a piston valve device for producing the damping force in a hydraulic shock absorber of type comprising a non-return valve element adapted to be opened during the compression stroke so as to permit the free flow of the working oil and a leaf valve element adapted to deflect downwards during the expansion stroke under the pressure of the working oil flowing through a plurality of holes formed through the non-return valve element, thereby producing the damping force, the improvement being capable of eliminating the variations in damping force, enhancing the durability of the leaf valve element and eliminating the shock produced when the non-return valve is closed, thereby offering a good riding quality when the hydraulic shock absorber is used on an automotive vehicle or a motorcycle.
The damping force producing piston valve device of the type described is disclosed in for instance U.S. Pat. No. 2,060,590, granted to J. E. Padgett, Nov. 10, 1936 and particularly in FIGS. 4 and 7 of the accompanying drawings thereof. However, the shock absorbing means disclosed therein has some defects to be described below.
During the compression stroke, the working fluid is forced upwardly through the passages 26 of the piston causing the flexible plate 29 (the leaf valve element) to be held in face to face contact with the rigid plate 28 (the non-return valve element) and causing both plates to move upwardly together away from the piston body. This lifting of the valve elements causes the rigid valve plate to separate from the annular seat, thereby allowing the working oil to flow upwardly around the outer edge of the rigid plate into the upper chamber 15 of the working cylinder. During the expansion stroke, the rigid valve plate 28 is seated against the piston body so that the working oil can be displaced downwardly through the piston only through the orifices of the rigid plate. The flexible plate 29 normally closes the orifices 32, but when the pressure acting on the flexible plate through the orifices increases sufficiently the flexible plate is sprung away from the rigid plate and a restricted displacement of fluid downwardly into the chamber 16 takes place. This restricted transfer of working oil produces a shock absorbing action. Since the leaf or flexible valve element is movable relative to the valve stop or guide 33, a clearance must be left between the valve stop or guide and the leaf or flexible valve element. As a result, in the case of the expansion stroke, the leaf or flexible valve element is forced to displace itself radially by a distance equal to the clearance. This means that during every expansion stroke, the portion of the leaf of flexible valve element in contact with the ridge 53 of the shoulder 52 of the piston is shifted so that the variations of the damping force result. The variations in the damping force are pronounced especially when the difference between the outer and inner diameters of the leaf or flexible valve element is small.
The damping force variation problem described above may be overcomed by securely clamping the leaf or flexible valve element between the valve stop or guide and the piston. However, if the leaf or flexible valve element is clamped, it is forced to deflect upwards even during the compression stroke so that as the hydraulic shock absorber compresses and expands, the leaf or flexible valve element is forced to deflect it self upwards and downwards. As a result, the earlier breakdown of the leaf or flexible valve element due to fatique results.
Furthermore, there is another problem. That is, when the compression stroke changes to the expansion stroke so that the non-return or rigid valve element is closed, the flow of the working oil is cut off instantaneously even though for a very short time interval until the downward deflection of the leaf or flexible valve element starts, thus resulting in shock causing the degradation of the riding quality.